1. Technical Field of the Invention
The present invention relates to a substrate temperature control method and device in a thin-film forming step in the manufacture of semiconductor elements, liquid crystal display panels or solar cells etc., or plasma processing apparatuses used in micro-processing steps.
2. Description of Related Art
In recent years, in plasma processing apparatuses, in order to achieve higher device functionality and lower processing costs, great efforts are being made to achieve higher precision, higher speeds, increase of area, and lower damage rates. In this connection, it is desired in particular to achieve uniform and precise control of the substrate temperature over its surface in order to obtain uniformity of film quality within the substrate during deposition and to ensure dimensional accuracy in the dry etching which is used in micro-processing. In order to achieve this, mechanical clamps or electrostatic attracting electrodes are employed as means for controlling substrate temperature and plasma processing apparatuses have begun to be used in which cooling is performed by introducing heat-conducting gas between the substrate and substrate holder.
A plasma processing apparatus using a conventional substrate temperature control device is described below. FIG. 2 shows the reaction chamber of a plasma processing apparatus constituting an example of the prior art. In FIG. 2, 101 is a vacuum chamber having means for reactive gas supply 130 and means for vacuum evacuation 131, 102 is an item to be treated or a substrate such as a silicon wafer, and 103 is an electrostatic attraction-type substrate holder comprising an alumina dielectric part 104 of thickness 5 mm and an aluminum base part 105 provided with a cooling water passage (not shown) in its interior. A pair of internal electrodes 106A, 106B for providing electrostatic attraction and consisting of tungsten are embedded 500 .mu.m within the outer surface of alumina dielectric part 104. A substrate push-up mechanism 120 is provided for substrate feed purposes within substrate holder 103. 121 is a spacer made of ceramics which electrically insulates vacuum chamber 101 and substrate holder 103. Holes for supplying heat-conductive gas are provided on the face of substrate holder 103 that contacts substrate 102. In this example, holes of diameter 1 mm are regularly arranged at five locations.
107 is a high frequency filter, 108 is a positive electrode DC power source, 109 is a negative electrode DC power source, 110 is a capacitor, 111 is a high frequency power source of frequency 13.56 MHz, and 112 is a grounded upper electrode.
113 is means for heat-conductive gas supply that supplies heat-conductive gas such as He to the gap between the upper surface of substrate holder 103 and the under-surface of substrate 102, comprising a valve and flow rate controller. 114 is a vacuum meter for monitoring the pressure of the heat-conductive gas at the under-surface of substrate 102; the pressure of the heat-conductive gas is controlled by an automatic pressure control valve 115 controlled by a signal from this vacuum meter 114. The flow rate of heat-conductive gas is changed in steps by means of a mass flow controller 116 and constructed so as to supply heat-conductive gas in a short time into a reservoir space comprising piping.
The operation of the plasma processing apparatus constructed as above will now be described. First of all, vacuum chamber 101 is evacuated to vacuum and substrate 102 is arranged on substrate holder 103; by applying positive and negative DC voltages of 1.0 kV from respective DC power sources 108 and 109 through high-frequency filters 107 to the pair of internal electrodes 106A and 106B, substrate 102 is electrostatically attracted on to substrate holder 103.
Next, He gas is supplied to the under-surface of substrate 102 by means for heat-conductive gas supply 113 and is regulated in pressure by automatic pressure control valve 115 and vacuum meter 114 for pressure monitoring at the under-surface of substrate 102. Vacuum meter 114 is set to a pressure such as to maintain attraction of substrate 102 on to substrate holder 103; in this case the pressure is controlled to 2000 Pa. When He gas is supplied by mass flow controller 116, cut-off valves 140, 141 are opened in order to raise the pressure in the gap between substrate holder 103 and substrate 102 to a set value. He gas flows from the holes in the surface of the substrate holder 103 contacting substrate 102 through He gas supply line 118.
Next, vacuum meter 114 for pressure monitoring at the under-surface of substrate 102 controls the pressure of the heat-conductive gas to a set value by regulating the pressure by opening and closing automatic pressure control valve 115. In the initial condition where the pressure is low, mass flow controller 116 permits a flow of He gas of 50 sccm; when the pressure rises to the set value of 2000 Pa, the flow rate of He gas drops to 30 sccm.
After this, the reaction gases CF.sub.4 at 30 sccm and O.sub.2 at 5 sccm are simultaneously introduced from means for reactive gas supply 130 and regulated to a pressure of 30 Pa by means for vacuum evacuation 131. A plasma is generated by branching the high-frequency power from high-frequency power source 111 into two, these being supplied to the pair of internal electrodes 106A and 106B through capacitors 110 that cut off the DC voltage. The required dry etching is thus performed whilst efficiently cooling substrate 102 from the under-surface using He gas.
When plasma processing has been completed, mass flow controller 116 is stopped, cut-off valve 140 is closed, the heat-conductive gas is evacuated through an evacuation line 119, and the pressure is lowered by fully opening automatic pressure control valve 115 until the pressure of the gap between substrate holder 103 and substrate 102 reaches the pressure in the initial condition. Substrate 102 is then lifted off from substrate holder 103 by means for pushing-up 120.
However, there are the following problems with the above prior art construction. As mentioned above, in order to supply heat-conductive gas in a short time into the reservoir space containing the piping, heat-conductive gas is delivered by mass flow controller 116 at 50 sccm in the initial low-pressure condition, and the pressure is regulated by dropping to 30 sccm when the pressure rises to 2000 Pa. This upper limiting value of the flow rate i.e. 50 sccm is determined by considering a flow rate such that attraction between substrate holder 103 and substrate 102 is not released and a flow rate such that dust is not entrained into the gap between substrate holder 103 and substrate 102 by the gas flow.
Since the upper limiting value of the supply flow rate of the heat-conductive gas was thus restricted, there was the problem that a long time was required before the pressure of the gap between the substrate holder 103 and substrate 102 could be raised to the set value.
A further problem was that the evacuation time after completion of plasma processing was also long, owing to the large evacuation resistance of automatic pressure control valve 115 when the pressure was lowered by fully opening automatic pressure control valve 115.
The conventional plasma processing apparatus therefore suffered from the problems of generation of dust at the under-surface of the substrate, or lowering of through-put.
It should be noted that means for heat-conductive gas supply/evacuation in respect of the gap between the substrate holder 103 and substrate 102 has been proposed in which the time required for supply of heat-conductive gas can be shortened, or it can be made possible to control the pressure to different values in different regions, by providing two or more substrate temperature control devices comprising a valve and flow rate controller. However, there is the problem of high costs and it is necessary to effect adjustment when performing gas pressure regulation, since the conductances of the supply lines cannot be made exactly identical. Also, if different regions are controlled to different pressures, there are problems such as that uniform etching cannot be achieved due to pressure differences if a substrate holder, whereby pressure can be raised to for example 3000 Pa, is employed.
Furthermore, means have been proposed wherein the supply line and evacuation line are constituted as a supply/evacuation direct line without connecting to the gap between the upper surface of substrate holder 103 and the under-surface of substrate 102, respectively, the line being branched after pressure regulation of the heat-conductive gas and connected to the gap between the substrate holder 103 and substrate 102, thereby reducing costs by simplification of the lines of the piping, but there is the problem that, even if some abnormality occurs in the gap between the surface of the substrate holder 103 and substrate 102, such as for example blockage of the holes for heat-conductive gas supply which are provided in the upper surface of substrate holder 103 by a foreign body of the substrate under-surface, mechanical abnormality cannot be detected.